Turning to FIG. 1, an example of a conventional comparator 100 can be seen. This comparator 100 generally comprises a differential input pair Q1 and Q2 (which are NMOS transistors), current mirror (which is generally comprised of PMOS transistors Q3 and Q4), current source 102, and output circuit 106 (which generally includes PMOS transistor Q3 and current source 104). In operation, a reference voltage REF is applied to the gate of transistor Q1, while an input signal IN is applied to the gate of transistor Q2. Based on a comparison of the input signal IN to the reference voltage REF, the output circuit 106 will provide an output signal that is logic high (“1”) or logic low (“0”).
In FIG. 2, an example of the operation of comparator 100 can be seen. For this example, the reference voltage REF is 0.75V, while the input signal IN is linearly increased or ramped. When the input signal IN is less than 0.75V, the output signal VOUT is logic high (i.e., about 1.5V), but once the input signal become greater than 0.75V, the output signal VOUT transitions to logic low (i.e., about 0V).
There are, however, other transistors that exhibit behaviors that are different from bipolar or unipolar transistors; these transistors are ambipolar transistors. Some examples of ambipolar transistors are graphene transistors, carbon nanotube (CNT) transistors, tunneling FETs (TFETs), and organic FETs (OFETs). As shown in FIG. 3, ambipolar transistors have an unusual gate response (where the drain current ID is large when the gate-source voltage VGS is low). As a result, it is generally unreasonable to replace bipolar or unipolar transistors with ambipolar transistors in conventional circuits (i.e., comparator 100) and expect that these conventional circuits will exhibit the same or similar behavior.
Therefore, there is a need for new circuits that take advantage of the characteristics of ambipolar transistors.
Some examples of uses for ambipolar transistors are: U.S. Patent Pre-Grant Publ. No. 2010/0065833; and U.S. Patent Pre-Grant Publ. No. 2010/0214034.